dts: arm: nrf51: add hfxo binding and nodes

Add nrf51 series hfxo binding and nodes to socs.

Note that the values added to the soc .dtsi files are worst case
defaults, which will be replaced with optimal values at board level
in the future, as they depend on the specific crystal in use.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>